The overall objective of the research planned and described is a systematic and detailed investigation of the interaction between early experience and genetic variables in the C57BL/10 mouse strain in an attempt to develop a more complete understanding of the mechanisms mediating between nature and nurture in the development of behavior. Specifically, the experiments are designed to determine whether the C57BL/10 uterine environment and/or X-chromosomal genes are required for effects of different early experiences in a variety of behavioral and biological traits. In the 1979-80 grant year, the following experiments are planned: 1) A study of the effects of fostering an open field activity and passive avoidance learning in the reciprocal F1 hybrids of DBA/1 and C57BL/10 mice to determine whether the X-chromosome and/or uterine environment of the C57BL/10 is required for this developmental lability. 2) If there are strain-by-treatment interactions for effects of fostering and/or early handling in the glucocorticoid stress response, the effect of this treatment on the glucocorticoid stress response of reciprocal F1 hybrids of DBA/1 and C57BL/10 mice will be assessed. 3) A study of the effects of fostering in control and ova transfer mice will be initiated to determine whether the C57BL/10 uterine environment and/or X-chromosomal gene(s) are required for the developmental lability of C57BL/10 aggressive behavior.